The present invention relates to a gas generator for an air bag for protecting a passenger from an impact, and an air bag system. In particular, this invention is concerned with a gas generator for an air bag wherein the ratio (A/At) of the total surface area A of solid bodies of gas generating agent contained in a housing to the total opening area At of gas discharge ports formed through the housing is controlled to a specified range.
In a conventional gas generator for an air bag, igniting means that is actuated when an impact sensor detects an impact, a gas generating agent that is ignited by the igniting means and burned to generate combustion gas, and filter means for cooling the combustion gas and/or scavenging combustion residues are accommodated in a housing having gas discharge ports. In this type of gas generator, when the igniting means is actuated upon detection of an impact, the gas generating agent is ignited and burned to generate combustion gas. The combustion gas is cooled and purified by the filter means in the housing, and discharged from the housing through gas discharge ports. Gas generating agents used for generating the combustion gas may be roughly classified into azide-containing gas generating agents, and other gas generating agent containing no azide.
The azide-containing gas generating agent (such as NaN3/CuO) has a relatively high linear burning velocity, for example, about 45-50 mm/sec under a pressure of 70 kg/cm2. Accordingly, even when the gas generating agent is formed into a relatively large pellet-like shape or disc-like shape that can be maintained with high stability, the gas generating pellets or discs may be completely burned in a desired period of time, i.e., 40 to 60 msec, when used in the gas generator for an air bag installed on the side of a driver seat, for example.
On the other hand, the non-azide gas generating agent generally has a linear burning speed of 30 mm/sec or lower. If this gas generating agent is formed into a pellet-like shape with a diameter of 2 mm, or a disc-like shape with a thickness of 2 mm, for example, the shape of the gas generating pellet or disc can be maintained with high stability, but it takes as much as about 100 msec to burn the gas generating agent where its linear burning velocity is about 20 mm/sec, which is longer than a desired burning time of 40 to 60 msec. Where the linear burning velocity is around 20 mm/sec, the diameter of the gas generating pellet or the thickness of the gas generating disc must be controlled to be around 1 mm to achieve a desired burning time. Where the linear burning velocity is 10 mm/sec or lower, the thickness of the gas generating pellet or disc must be reduced to 0.5 mm or smaller. It is, however, practically impossible to produce pellets or discs of the gas generating agent having such diameter or thickness, which can withstand vibrations of an automobile for a long period of time, while being held in an industrially stable condition. The gas generator containing such gas generating pellets or discs does not perform its functions satisfactorily. Thus, it has been difficult to develop a gas generator that contains a non-azide gas generating agent and can be advantageously used in practical applications.
It is, therefore, an object of the present invention to provide a gas generator for an air bag which permits its gas generating agent to be completely burned within a desired period of time, and also shows satisfactory operating characteristics.
Since the maximum pressure in the housing upon actuation of the gas generator varies with the temperature of the outside air or atmosphere, it is difficult to provide a gas generator for an air bag which exhibits stable operating characteristics, and does not substantially depend upon the temperature of the atmosphere.
It is, therefore, another object of the present invention to provide a gas generator for an air bag which can be manufactured at a reduced cost, and which is able to operate with high stability, without depending upon the temperature of the atmosphere.
The gas generator for an air bag according to the present invention is characterized in that the ratio (A/At) of the total surface area A of solid bodies of gas generating agent stored in a housing to the total opening area At of gas discharge ports formed through the housing is controlled to a specified range.
More specifically, in the gas generator for an air bag of the present invention, wherein igniting means that is actuated when an impact sensor detects an impact, a gas generating agent that is ignited by the igniting means and burned to generate combustion gas, and filter means for cooling the combustion gas and scavenging combustion residues are accommodated in a housing having gas discharge ports, the ratio (A/At) of the total surface area A of solid bodies of the gas generating agent to the total opening area At of the gas discharge ports is controlled to be larger than 300.
With the ratio (A/At) of the total surface area A of the gas generating agent to the total opening area At of the gas discharge ports being controlled to be larger than 300, a difference between maximum output pressures at 85xc2x0 C. and 20xc2x0 C. and a difference between maximum output pressures at 20xc2x0 C. and xe2x88x9240xc2x0 C. in tank pressure tests are respectively not higher than 40 kPa. In the gas generator for an air bag to be used for a driver seat side and a passenger seat side, the ratio (A/At) of the total surface area A of the gas generating agent to the total opening area At of the gas discharge ports may be controlled to be larger than 300, but not larger than 1300, and preferably controlled to be in a range of 450 to 1300, more preferably, in a range of 450 to 1000.
The above-explained housing may be formed by casting or forging, or may be formed by pressing a diffuser shell having gas discharge ports through which the gas generated by burning the gas generating agent is discharged, and a closure shell having a central aperture in which the igniting means is disposed, and joining these shells together by various welding methods, such as plasma welding, friction welding, projection welding, electron beam welding, laser welding and TIG welding. The housing thus formed by press working can be easily manufactured at a reduced cost. Each of the diffuser shell and closure shell may be formed from a stainless steel sheet having a thickness of 1.2 to 3.0 mm, for example. The volume content of the housing is desirably in the range of 60 to 130 cc. The gas discharge ports formed through this housing are desirably circular holes, each having an inside diameter of 2 to 5 mm, and the total opening area of these discharge ports is desirably in a range of 50 to 200 mm2 where the gas generator is used for an air bag for a driver seat side, and in a range of 60 to 500 mm2 where the gas generator is used for an air bag for a passenger seat side.
The gas discharge ports of the housing are preferably closed by an aluminum tape having a width that is 2 to 3.5 times the diameter of each discharge port, for inhibiting entry of moisture from the exterior space into the housing. The aluminum tape may be an adhesive aluminum tape, or may be attached to the housing by means of various kinds of adhesives, such as those that are fused by heat to provide secure bonding. For example, a hot melt adhesive may be used to attach the aluminum tape to the housing.
The gas generating agent is more effectively used in the present gas generator particularly when its linear burning velocity is in the range of 7 to 30 mm/sec, preferably 7 to 15 mm/sec, under a pressure of 70 kg/cm2. The gas generating agent having such a property may be a non-azide gas generating agent containing a nitrogen containing organic compound, an oxidizing agent, and a slag-forming agent, for example. The content of the nitrogen containing compound in the gas generating agent may be in the range of 25 to 60% by weight, and the content of the oxidizing agent may be in the range of 40 to 65% by weight, while the content of the slag-forming agent may be in the range of 1 to 20% by weight.
The nitrogen containing compound serves as a fuel and a nitrogen source. Such a nitrogen containing compound may be selected from those containing tetrazole, triazole, or a nitrogen containing organic compound of these metallic salts or the like, and an oxygen containing oxidizing agent, such as alkali metal nitrate, as major components, and triaminoguanidine nitrate, carbohydrazide, nitroguanidine and others. In the present invention, nitroguanidine is particularly preferred. The content of the nitrogen containing compound in the gas generating agent may be generally in the range of 25 to 60% by weight, preferably, in the range of 30 to 40% by weight, though it varies depending upon the number of carbon elements, hydrogen elements and other oxidized elements in its molecular formula. Although the absolute value of the content of the nitrogen containing compound differs depending upon the type of the oxidizing agent used, minor CO concentration in the generated gas increases as the absolute value is larger than the complete oxidation theoretical value, and minor NOx concentration in the generated gas increases as the absolute value is equal to or smaller than the complete oxidation theoretical value. Accordingly, the content of the nitrogen containing compound is most preferably controlled in the range in which these concentrations are optimally balanced.
The slag-forming agent in the gas generating composition functions to convert a liquid form of an oxide of alkali metal or alkali earth metal particularly produced by decomposition of the oxidizing agent in the gas generating composition, into a solid form, so as to retain the oxide in the combustion chamber and thus prevent the oxide in the form of mist from being discharged out of the inflator. The optimum slag-forming agent may be selected to be suited for the metallic component to be converted into the solid form. The slag-forming agent may be formed of at least one kind selected from natural clays containing aminosilicate as a major component, such as those of bentonite and kaolin, and artificial clays, such as synthetic mica, synthetic kaolinite, and synthetic smectite, and talc as one kind of minerals of water-containing magnesium silicate. In the present invention, acid clay may be preferably used as the slag-forming agent. The content of the slag-forming agent in the gas generating agent may vary from 1 to 20% by weight, and is preferably in the range of 3 to 7% by weight. If the content of the slag-forming agent is too large, the linear burning velocity is reduced, with a result of reduction in the gas generating efficiency. If the content is too small, the slag-forming agent cannot sufficiently fulfill its slag-forming function.
The oxidizing agent may be selected from nitrates of alkali metals or alkali earth metals, chlorates, and perchlorates, as well known in the art. In particular, the oxidizing agent preferably comprises at least one kind selected from nitrates of alkali metals or alkali earth metals, which contain cation. For example, strontium nitrate is preferably used. Although the absolute value of the content of the oxidizing agent in the gas generating agent varies depending upon the kind and amount of the gas generating compound used, it is preferably in the range of 40 to 65% by weight, in particular, in the range of 45 to 60% by weight in view of the CO and NOx concentrations as described above.
The gas generating agent may further contain a selected one of various known binders, where the gas generating agent is formed into a certain shape to provide solid bodies.
The gas generating agent may be formed into a cylindrical shape with a single hole, and in this case, the surface area of each body of the gas generating agent can be increased. To enable the gas generating agent to be completely burned within a desired burning time, the smallest thickness of the cylindrical wall of the gas generating body is preferably controlled to 0.01-2.5 mm, more preferably, 0.01 to 1.0 mm. Where this thickness is 0.85 mm, for example, the cylindrical gas generating body with a single hole may have an outside diameter of 2.5 mm and an inside diameter of 0.8 mm. Where the thickness is 1.2 mm, the cylindrical gas generating body with a single hole may have an outside diameter of 3.2 mm and an inside diameter of 0.8 mm.
The amount of the gas generating agent contained in the gas generator is preferably in a range of 20 to 50 g where the gas generator is to be used for an air bag for a driver seat side, and in a range of 50 to 190 g where the gas generator is to be used for an air bag for a passenger seat side.
Where the gas generating agent contained in the housing is a non-azide type gas generating agent, it has a linear burning velocity of 5 to 30 mm/sec under a pressure of 70 kg/cm2. When this gas generating agent is used in gas generators for air bags installed on an automobile, the gas generating agent needs to be completely burned in 40 to 60 msec to inflate an air bag for a driver seat side, and in 50 to 80 msec to inflate an air bag for a passenger seat side. To control combustion of the gas generating agent, therefore, the ratio (A/At) of the total surface area A of solid bodies of the gas generating agent to the total opening area At of the gas discharge ports is controlled to be larger than 300. In the gas generator for air bags used on the side of the driver seat and passenger seat, this ratio A/At may be controlled to be larger than 300, but not larger than 1300, and preferably controlled to be in the range of 450 to 1300, more preferably, in a range of 450 to 1000. In this case, the gas generating agent can be completely burned in the above-indicated period of time.
If the ratio A/At exceeds the maximum value, the pressure in the gas generator excessively increases, and the burning velocity of the gas generating agent becomes excessively high. If the ratio A/At is less than the minimum value, on the other hand, the pressure in the gas generator is lowered, and the burning velocity becomes excessively low. In either case, the burning time of the gas generating agent is outside the desired range, and an operable gas generator cannot be provided.
When the ratio (A/At) of the total surface area A of the gas generating agent to the total opening area At of the gas discharge ports is controlled as described above, a difference between maximum pressures at 85xc2x0 C. and 20xc2x0 C. and a difference between maximum pressures at 20xc2x0 C. and xe2x88x9240xc2x0 C. in tank pressure tests are respectively not higher than 40 kPa. Thus, the maximum pressure in the housing upon actuation of the gas generator does not depend upon the temperature of the atmosphere, and the gas generator for an air bag according to the present invention exhibits stable operating characteristics.
In the tank pressure test, the gas generator containing the shaped bodies of gas generating agent is fixed to the inside of a tank made of SUS (stainless steel: according to Japanese Industrial Standard) and having a content volume of 60 liters. After the tank is air-tightly closed, the gas generator is connected to an external ignition circuit. By using a pressure transducer installed in the tank, pressure increases or changes in the tank are measured from time 0 to 200 milliseconds where the time 0 indicates a point of time when the switch of the ignition circuit is turned on. Measurement data are processed by a computer, and finally represented as a tank pressure/time curve from which operating characteristics of the gas generator can be evaluated. In this test, a portion of the gas in the tank may be sampled out after the combustion, to be analyzed in respect of its CO and NOx components, for example. According to the present invention, the tank pressure tests are conducted at xe2x88x9240xc2x0 C., 20xc2x0 C., and 85xc2x0 C., to obtain the maximum output pressure (namely, maximum tank pressure) from the tank pressure/time curve at each temperature, and the difference between the maximum pressures in the tank pressure tests at 85xc2x0 C. and 20xc2x0 C. and the difference between the maximum output pressures in the tank pressure tests at 20xc2x0 C. and xe2x88x9240xc2x0 C. are calculated.
The filter means removes combustion residues produced by combustion of the gas generating agent, and cools the combustion gas. As the filter means having these functions, a conventionally used filter for purifying the generated gas and/or a coolant for cooling the generated gas may be used, or a layered wire screen filter or the like may be used which is obtained by compressing wire screens formed of a suitable material into an annular layered structure. This layered wire screen filter may be formed by superposing some layers of plain-stitch wire mesh screens in radial directions, and compressing the wire screens in the radial and axial directions into an annular shape. The filter means thus formed has a complicated porous structure, and provides an excellent scavenging effect. Thus, the filter means provides a coolant/filter unit having both cooling and scavenging functions. More specifically, a plain-stitch wire screen made of a stainless steel is formed into a cylindrical body, and one end portion of this cylindrical body is repeatedly bent outward to form an annular layered body, which is then compressed in a mold to form the filter means. In another method, a plain-stitch wire screen or mesh sheet made of stainless steel may be formed into a cylindrical body, and the cylindrical body is pressed in a radial direction and formed into a planar body, which is then rolled cylindrically many times into a multi-layered body. This layered body is then compressed in a mold to thus form the filter means. The stainless steel used as a material for the wire screens may be selected from SUS 304, SUS 310S, SUS 316 (according to Japanese Industrial Standard). In particular, SUS 304 (18Cr-8Ni-0.06C), which is austenitic stainless steel, shows excellent corrosion resistance.
The filter means may have a dual-layer structure having an inner or outer layer consisting of the layered wire-screen body. The inner layer may have a function of protecting the filter means from the combustion gas generated by combustion of the gas generating agent. The outer layer may have functions to prevent the filter means from swelling or expanding out due to the gas pressure upon actuation of the gas generator, and closing a space formed between the filter means and an outer circumferential wall of the housing. Where the filter means is spaced apart from the inner circumferential surface of the housing, namely, where the space is formed between the outer circumferential surface of the filter means and the inner circumferential surface of the housing, this space functions as a gas passage, which permits the combustion gas to pass through the entire region of the filter means such that the gas is effectively cooled and purified during the passage.
The terms xe2x80x9ccoolant/filterxe2x80x9d and/or xe2x80x9ccoolant-filterxe2x80x9d used herein refer to a filter means which both cools combustion gases and scavenges residues therefrom.
The gas generator for an air bag according to the present invention may employ any type of system for sensing an impact and actuating the igniting means provided that the gas generator is constructed as described above. Namely, the impact sensing and igniting system may be of a mechanical ignition type in which the ignition means is actuated only by a mechanical arrangement to generate gas when an impact sensor detects an impact, or may be of an electric ignition type in which the igniting means is actuated in response to an electric signal transmitted from an impact sensor upon detection of an impact, to generate gas.
In the mechanical ignition type gas generator using mechanical igniting means, a mechanical sensor for sensing an impact only by a mechanical arrangement, for example, a sensor adapted to launch a plunger upon movement of a weight, is mounted in the housing. This housing is formed with a plurality of gas discharge ports, and incorporates a detonator that is ignited and burned when the plunger launched by the mechanical sensor enters the detonator, igniting means comprising a transfer charge that is ignited and burned by the flame of the detonator, gas generating means that is ignited and burned by the flame of the transfer charge to generate gas, and filter means for cooling and purifying the generated gas. In the electric ignition type gas generator using electrical igniting means, on the other hand, the housing having gas discharge ports incorporates igniting means comprising an igniter to be actuated in response to an electric signal transmitted from a sensor upon detection of an impact, a transfer charge that is ignited and burned upon actuation of the igniter, gas generating means that is ignited and burned by a flame produced by the transfer charge to generate gas, and filter means for cooling and purifying the generated gas. The gas generator of mechanical ignition type or electric ignition type may employ a suitably selected structure other than the above, which is advantageous in the operating characteristics.
The gas generator of the present invention may include elements other than those indicated above, provided that the ratio (A/At) of the total surface area A of the solid bodies of the gas generating agent installed in the housing to the total opening area At of the gas discharge ports is controlled as described above. For instance, the gas generator may employ a perforated cylindrical plate that surrounds the outer periphery of the filter means to inhibit deformation of the filter means, and short-pass preventing means (plate members etc.) for surrounding the upper end and/or lower end of the inner periphery of the filter means to inhibit the generated gas from short-passing through a clearance between the filter means and an inner surface of the housing. The gas generator may also include a perforated basket having a cylindrical shape and many holes, which surrounds the inner periphery of the filter means to prevent direct contact between the gas generating means and the filter means. The gas generator for an air bag, as described above, and the air bag that is inflated by the gas generated by the gas generator are accommodated in a module case, to provide an air bag system. This air bag system further includes an impact sensor for detecting an impact to actuate the gas generator. If the gas generator is of a mechanical ignition type, the impact sensor consists of a mechanical sensor, and is stored in the housing, along with the igniting means. If the gas generator is of electric ignition type, on the other hand, the impact sensor may consist of a semiconductor acceleration sensor disposed outside a console box, for example. In this semiconductor type acceleration sensor, four semiconductor strain gauges are formed on a beam of a silicon substrate that is adapted to deflect upon application of acceleration, such that these strain gauges are connected in a bridge form. The beam is deflected upon application of acceleration thereto, and strains occur on the surface of the beam. The resistance of the semiconductor strain gauges changes due to the strains, and the changes in the resistance are detected as voltage signals that are proportional to the applied acceleration. Where the electric ignition type gas generator is used, in particular, the air bag system may also include a control unit provided outside the module case. This control unit is provided with an ignition determining circuit, which receives signals from the semiconductor type acceleration sensor. At the point of time when the impact signal from the impact sensor exceeds a given value, the control unit starts computing, and generates an actuation signal to the gas generator when the result of computing exceeds a given value.
In this air bag system, the gas generator is actuated in association with sensing of an impact by the impact sensor to discharge the combustion gas through its gas discharge ports. The combustion gas is expelled into the air bag to inflate the air bag while breaking the module cover, so that the inflated air bag forms a cushion between a hard structure in the vehicle and a passenger.